Tail rotor blade

ABSTRACT

A tail rotor blade for a helicopter has a core 22 whose shape defines the trailing portions of the blade. Core 22 is adhered to upper 20 and lower 21 metallic skins which extend within and are adhered to the inner surfaces at the trailing edge of a metallic nose spar 10. A pair of straps 30, 31 of unidirectional fiberglass strands extend spanwise and are adhered to the inner surface of the leading edges of the skins 20, 21 and the outer leading surfaces of the core 22 as to span chordwise the regions of the joints between nose spar 10 and skins 20, 21.

TECHNICAL FIELD

This invention relates to a tail rotor blade for a helicopter and moreparticularly to a tail rotor blade having leading edge and trailing edgereinforcement by unidirectional spanwise fiberglass strapping integratedwith the blade core and skins.

BACKGROUND ART

Tail rotor blades heretofore have been provided in a variety ofdifferent specific constructions, generally with a honeycomb core towhich upper and lower trailing edge skins are attached and which in turnare secured to a nose spar.

In U.S. Pat. No. 4,120,610 (U.S. application Ser. No. 470,520 filed May16, 1974) there is disclosed a main rotor blade system in which spanwisefiberglass bands are secured to the inner surface of the nose sparassembly to enhance the ability of the blade to withstand centrifugalforces and to prevent catastrophic conditions when the blade developscracks in metallic portions thereof.

DISCLOSURE OF THE INVENTION

The present invention is directed to a tail rotor blade wherein ametallic honeycomb core has a shape defining the trailing portions ofthe blade with upper and lower skins adhered to the core and with astainless steel leading edge spar. Upper and lower unidirectionalfiberglass straps are adhered to the leading edge upper and lowersurfaces of said core and to the inner leading surfaces of said skinsand with the outer leading edge surfaces of the skins being adhered tothe inner trailing surfaces of the nose spar. A spanwise strap issecured between the inner trailing surfaces of the skins.

In a further aspect, the first mentioned fiberglass straps are madeprogressively thinner as they extend from the root to the tip of saidrotor blade.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of one embodiment of the present invention;

FIG. 2 is a sectional view taken along the lines 2--2 of FIG. 1; and

FIG. 3 illustrates an embodiment of a graded fiberglass strap.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIG. 1 a helicopter tail rotor blade has been showncomprising a nose spar 10 which extends from an inboard or root couplingportion 11 to the outboard end 12. A trailing section 13 extends fromthe nose spar to the trailing edge 14.

In accordance with the present invention the rotor blade is providedwith three unidirectional fiberglass straps which extend spanwise of theblade.

More particularly a first pair of straps, upper and lower straps, occupythe zones beneath the boundary between the nose spar 10 and the trailingskins 13, the zones being indicated by the dotted lines 15. A thirdstrap occupies the zone between the trailing edge surfaces of thesection 13, the zone being indicated by the dotted line 16.

Referring to FIG. 2 which is a cross sectional view of the blade in FIG.1 it will be seen that the nose spar 10 forms the leading edge of theblade and extends to lines 10a and 10b which mark the trailing edges ofthe nose spar 10. Trailing edge skins 20 and 21 extend beneath and aresecured to the inner surfaces of the trailing edges of the nose spar 10.The skins 20 and 21 are adhered to a honeycomb core 22, the core 22being so formed as to define the shape of the blade from the leadingedge 23 of core 22 and to the trailing edge 24 of core 22.

In accordance with the present invention, three unidirectionalfiberglass straps are provided. The first strap 30 is adhered to theinside of skin 20 in a region which overlaps the trailing edge 10a ofnose spar 10. The strap 30 extends to the leading edge of the core 22.The second strap 31 is secured inside of skin 21 where it underlies thetrailing edge 10b of nose spar 10. Strap 31 extends forward to theleading edge of the core 22.

A third strap 32 is secured between the trailing edges of the skins 20and 21 and is positioned immediately behind the trailing edge 24 of core22.

The blade thus far described provides a unit of design to avoidcatastrophic failure modes and provides for longer service life. Thusthe blade is constructed of a stainless steel spar, preferably stainlesssteel skins and an aluminum honeycomb core. The trailing edge strip isconstructed of fiberglass unidirectional plys which are orientedunidirectionally spanwise to act as a crack stopper in the event that askin crack starts propagating toward the trailing edge of the blade. Thefiberglass trailing edge strap is precured and then bonded between theskins during a final bonding operation. In addition, the unidirectionalfiberglass straps 30 and 31 are also unidirectional spanwise and areinstalled under the skins and the spar at the spar-to-skin joint toprevent cracks propagating either through the spar or through the skins.Th straps 30 and 31 are of graded thickness decreasing from root to tip.The change in thickness is obtained by dropping off plys of thefiberglass toward the tip of the blade. The straps 30 and 31 areco-cured to the blade during a final bonding operation.

In a preferred embodiment of the invention, that portion of the nosespar at the tip lying forward of line 40 is plated with nickel, thethickness of which is substantial at the tip and which is of zerothickness at the point 41. It is also of zero thickness along line 40but of maximum thickness as it appears at the tip on the leading edge ofthe nose spar 10. Nickel plating accommodates the high erosion rate atthe tip and tapers to zero thickness cordwise.

In prior art systems, a crack in a blade can propagate at such a fastrate that a failure will occur before found during routine inspections.A blade of the present invention avoids that possibility.

Referring now to FIG. 3, strap 30 is shown in simplified form for usewith a blade of length of about 53 inches. The strap 30 is shown asformed of a multiple fiberglass tape of the type manufactured and soldby the 3M Company of Minneapolis, Minnesota, and identified as SP 250-S2Scotch ply fiberglass tape. In a preferred embodiment, in the regionfrom the root to about 18 inches, straps 30 and 31 are each made up offour plys, having a thickness of 0.080 inches. From 18 inches out toabout 34 inches the straps 30 and 31 are made up of three plys with athickness of 0.060 inches. From 34 inches to the tip of the blade thestraps 30 and 31 are two-ply straps having 0.040 inches thickness. Inthe embodiment herein disclosed the straps 30 and 31 have a width ofabout two inches. It is noted that the plys as they terminate are shapedto a point.

The trailing edge strip 32 is preferably of uniform thickness from rootto tip and of a width of about 0.80 inches and of two-ply constructionhaving a thickness of 0.040 inches.

Having described; what is claimed is:
 1. A tail rotor blade for ahelicopter which comprises:(a) a metallic nose spar; (b) a core whoseshape defines the trailing portions of the blade; (c) upper and lowermetallic skins which extend within and are adhered to the inner surfacesat the trailing edges of said metallic nose spar and to said core; and(d) a pair of straps of unidirectional fiberglass strands extendingspanwise and adhered to the inner surface of the leading edges of saidskins and the outer leading surfaces of said core as to span chordwisethe region of the joint between said nose spar and said skins.
 2. Thecombination set forth in claim 1 in which a unidirectional fiberglassstrap with the fiberglass strands extending spanwise is adhered betweenthe inner surfaces of the trailing edges of said skins.
 3. Thecombination set forth in claim 1 in which the thickness of said strapsprogressively decreases from root to tip.
 4. The combination set forthin claim 1 in which said straps are about twice as thick at the root asat the tip.
 5. The combination set forth in claim 4 in which said strapshave a maximum thickness of about 0.080 inches at the root and a minimumthickness of about 0.040 inches at the tip.